Prior Art
Since the introduction of motor vehicles, the market demand for automatic transmissions has grown strong due to their obvious user benefits over the standard manual gearshift/pedal clutch transmission.
Despite extensive development efforts in the automatic transmission field, state-of-the-art automatic transmissions, which rely largely on multi-stage synchronous clutch shifting between various mechanical/hydraulic power split modes, remain substantially less efficient and more costly than standard manual gearshifts, and frequently suffer from a marginal level of uneven shifting characteristics as well as reliabilty and serviceability shortcomings, due in part to high complexity.
Paradoxically, standard manual gearshifts, which demand a wide variation of engine speed at each shift, remain substantially superior to automatic gearshifts in efficiency and fuel economy, even though a truly continuously-variable automatic transmission has the potential of equalling and even surpassing the fuel economy of manual gearshifts, especially under stop-and-go driving conditions, by keeping the engine speed constant during vehicle acceleration, provided the power losses normally associated with the hydraulic portion of automatic transmissions are reduced to a low level.
The great majority of known automatic transmissions are based on the principle of an elementary split type hydromechanical subsystem comprising a differential (usually planetary) gear set interposed between the input and output shafts, with its reaction gear element (reactor) coupled through an auxiliary bilateral variable-ratio device, typically a hydraulicly-coupled pair of variable-stroke hydrostatic motor-pumps, to either the transmission's input shaft or its output shaft, thus splitting the power flow, at either the input or the output, into a mechanical branch in which power is transferred through the two main gear elements of the planetary set, and a hydraulic branch in which a portion of the power flows into or out from the reactor gear element of the planetary set, through the variable-ratio device.
This basic subsystem is highly efficient at a base speed ratio, when the reactor is in effect locked against rotation and all of the power is transferred through the mechanical branch; however, at ratios other than the base ratio, with the reactor rotating, the portion of the total input power routed through the hydraulic branch increases and associated power losses increase with increasing separation between the base ratio and the actual operating ratio selected. For example, a total transmission ratio range of 2 requires 1/3 of the input power to flow through the hydraulic branch at each end of the range. This power flow entails appreciable power loss, depending on the efficiency of the hydraulic device. Generally the gear trains in the mechanical branch are highly efficient, so practically all power losses are attributable to the hydraulic branch.
Such power loss, along with practical limits in fluid pressure and power-handling capacity of the hydraulic branch, limit the range of speed and torque conversion so severely that in transmissions for motor vehicles it has been necessary to provide a plurality of such subsystems in progressive modes of differing input/output speed ratios, using combinations of clutches, gear trains, hydraulic control valves and other devices, to shift between the various modes and to uncouple the unused elements in a particular mode.
In U.S. Pat. No. 2,830,468, Waring discloses a transmission having an auxiliary variable-ratio device with its first shaft coupled to the reactor of an epicyclic gear train (planetary gear set), and its second shaft coupled to either the transmission's input or its output, as selected by means of a complementary pair of clutches synchronized to shift between split-input and split-output modes at zero reactor rotation speed. The Waring patent is particulary concerned with avoiding operation in power-regenerative operating modes where feedback power could circulate and increase losses and power-handling requirements to abnormal levels. Accordingly, the predominant principle and intent taught in Waring's patent is to clutch-shift the transmission into a split-input configuration for ratios below the base ratio, and into a split-output configuration for ratios above the base ratio, so that the transmission is never allowed to operate in other modes which depend on regenerative power feedback.
A two-mode hydromechanical transmission, shifting between a split-input and a split-output mode, is disclosed by Miyao et al in U.S. Pat. No. 3,869,939, employing a complementary pair or quartet of synchronized clutches to override the differential action of one or other of a tandem pair of planetary gear sets having differing base ratios, and thus select either a split-input mode at a first base ratio or a split-output mode at a second base ratio, providing a wider range of speed ratios than the Waring patent by allowing the system to operate in a power-regenerative condition over a portion of the total range.
An example of a multi-mode hydromechanical approach, utilizing combinations of the basic subsystems and general principles outlined above, is seen in U.S. Pat. No. 3,455,183 to Orshansky, in which three planetary gear sets, up to six synchronous clutch means and one or two dual hydraulic motor-pump variable-ratio devices are combined by gear and shaft means and commanded by control means including multiple cam-operated valves in hydraulic fluid lines, to enable automatic shifting between several progressive modes having different ratios and different power path configurations which may be entirely mechanical or split into a mechanical and a hydraulic branch at either the input or output shaft depending on the mode selected.
In the development of prior art automatic transmissions, a major issue involving a great deal of development effort has been the problem of mitigating unwanted shocks due to transient mechanical and hydraulic pressure gradients during the shifting transition between modes, especially under full load such as when accelerating a motor vehicle. For example, critical adjustments are required in synchronizing multiple mode-shifting clutches. Measures taken to smooth out these transitions to even a tolerable level are often less than fully successful in practice, and at best the most advanced multi-mode automatic transmissions represent a piecewise synthesized compromise which falls short of the ideal smooth performance potentially available from a clutchless automatic transmission operating over a single wide range of uniform continuously-variable ratio without range-shifting.